The present invention relates to a device for reduction of tensile forces occurring in a mooring hawser when a ship, particularly a tanker, is moored in a point to a loading buoy or the like having a rotatable buoy top, where a linkage having a torsion spring device is arranged between the buoy top and the attachment for the mooring hawser.
The object of the invention is to provide a reduction in the occasionally very high stress peaks occuring in the mooring hawser in a relatively simple manner, which do not require manning of the buoy or any other installation where the mooring hawser is attached. Even though the buoy is relatively small with respect to the ship, it will largely follow the wave motions. So will the ship but with a different phase, so that the distance between the buoy and the ship may vary considerably.
A method is known for reducing the hawser force when the loading buoy is of the slender column type having a rotatable top section, wherein one utilizes the inherent elasticity in a long hawser for dampening the force variations occurring when wind, waves and current act differently on ship and buoy. This is done by running the hawser along the center of the buoy via a vertically positioned sheave and further substantially horizontally through a trumpetlike hawsehole and to the ship. In this manner the hawser is always run down along the central axis of the buoy. However, this method is somewhat costly since the sheave and its support, as well as the hawser attachment and associated swivel must be dimensioned so as to withstand hawser forces approaching 600 metric tons. Furthermore, the hawser attachment point and swivel will be placed deep down into a shaft and therefore are not readily accessible for maintenance, replacement of the hawser, etc.
In another type of rotatable buoy top, which also is to be used in the North Sea, the hawser is run through a hawsehole and over a sheave in order to make the elastic hawser length substantially greater than the distance between the buoy top and the ship.
A major disadvantage in said buoy tops having hawseholes and sheaves for the hawser has been excessive wear of the hawser due to the contact with these elements. Furthermore, experience has shown that the elasticity of the hawser is substantially reduced after some use, and it is further redecued in the working condition due to wave frequencies giving relatively high velocities. These circumstances have resulted in several hawser failures.
It has also been suggested to utilize a hydraulic compensator for dampening the maximum load in the mooring hawser, but this concept has met with substantial scepticism due to the large amounts of energy pulsating in the hydraulic damping system. This energy may of course be dissipated by cooling, but it is not desirable to have a cooling water system on the buoy, which should be unmanned in the mooring condition.
The object of the invention is therefore to compensate the variation in the distance between the ship and buoy without providing additional elasticity in the hawser by making it longer than the distance between the buoy and the ship, and without utilizing a hydraulic compensating device.